Epistasis - Definition, Types, Examples, and Causes

Epistasis: Genes can also work against each other, where one gene stops another gene from being expressed. This is called epistasis. In epistasis, genes interact so that one gene hides or blocks another gene's expression. The term "epistasis" comes from Greek and means "standing upon." The genes that are being blocked or silenced are called hypostatic to the genes that are blocking them.

Epistasis often involves a gene pathway where one gene's expression relies on the function of a gene that comes before or after it in the pathway. The article below explains epistasis, including its definition, examples, types, and detailed NEET Biology Notes .

Epistasis Definition

Epistasis is a fundamental concept in genetics that describes how genes interact to influence trait expression. It is important to understand the structure and function of genetic pathways and the evolutionary dynamics of complex genetic systems.

Epistasis analysis is a method used to determine the order of gene action in a regulatory hierarchy. This method compares the phenotype of a double mutant to that of a single mutant to better understand how genes interact and influence trait expression.

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Epistasis in Genetics

Epistasis is a genetic phenomenon where the impact of a gene mutation is influenced by the presence or absence of mutations in one or more other genes, known as modifier genes. Simply put, the effect of the mutation depends on the genetic context in which it occurs. Epistatic mutations exhibit varying effects when present alone compared to when they occur together. Initially, epistasis specifically described how another gene's action hides the effect of a gene variant.

Okazaki Fragments

Types of Epistasis

Epistasis can occur in various ways, including:

1. Recessive Epistasis

This occurs when recessive alleles at one locus mask the expression of both dominant and recessive alleles at another locus. It's also known as supplementary epistasis.

Example: In Labrador retrievers, coat color is determined by genes at two loci. One locus determines pigment type (black or brown), and the other affects pigment deposition in hair shafts. The ee genotype at the second locus masks the expression of the alleles at the first locus, resulting in different coat colors.

Okazaki Fragments

2. Dominant Epistasis

This occurs when the expression of both dominant and recessive alleles at another locus is masked by a dominant allele at one locus.

Example: In summer squash, fruit color is controlled by genes at two loci. The W allele is dominant to w and masks the expression of the G/g alleles, resulting in different fruit colors.

3. Duplicate Recessive Epistasis

This occurs when recessive alleles at either of two loci can hide the expression of dominant alleles at both loci.

Example: In sweet pea plants, the purple flower color is controlled by two dominant genes (A and B). The presence of recessive alleles at either locus results in white flowers.

4. Duplicate Dominant Epistasis

This occurs when a dominant allele at one of two loci can hide the expression of recessive alleles at both loci.

Example: In rice, the development of awn (a bristle-like extension) is governed by two dominant duplicate genes (A and B). The presence of any of these alleles results in the development of awn, and the awnless condition appears only when both genes are in a homozygous recessive state (aabb).

Epistasis Ratio

Epistasis, the interaction of genes where one gene masks the effect of another gene, can manifest in various forms, including:

• Dominant Epistasis (12:3:1): A dominant allele at one gene locus masks the expression of alleles at another locus.
• Recessive Epistasis (9:3:4): A recessive allele at one gene locus suppresses the expression of alleles at another locus.
• Duplicate Recessive Epistasis (9:7): Requires two recessive alleles, one from each gene, for a specific phenotype to be expressed.
• Complementary Epistasis (9:7) : Requires at least one dominant allele from each gene for a specific phenotype. Both dominant alleles or both recessive alleles at both loci lead to different phenotypes.
• Dominant Inhibitory Epistasis (12:3:1): Similar to dominant epistasis, but the dominant allele inhibits the expression of another gene entirely, even its dominant form.

It's important to note that while these ratios are common, epistasis can exhibit greater complexity depending on the genes involved.

Sexual Reproduction in Flowering Plants

Epistasis Causes

Epistasis, the masking or modification of gene expression by other genes, can arise from several genetic mechanisms:

1. Gene Regulation: Mutations in genes that regulate other genes can disrupt normal expression patterns, causing epistasis.
2. Enzyme Function: Mutations in genes coding for enzymes in metabolic pathways can block the pathway, affecting downstream gene expression.
3. Protein-Protein Interactions: Mutations affecting interactions between proteins coded by different genes can disrupt normal functions, leading to epistasis.
4. Signalling Pathways: Mutations in genes coding for proteins in signalling pathways can disrupt signal transmission, affecting downstream gene expression.
5. Developmental Pathways: Mutations in genes involved in development can disrupt the normal sequence of gene expression, leading to epistasis.
6. Dosage Compensation: Disruption of dosage compensation mechanisms can lead to differences in gene expression between sexes, causing epistatic effects.

These mechanisms illustrate the diverse ways in which gene interactions can result in epistasis.

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Epistasis Examples

Epistasis is the complex interaction of genes that affects an organism's phenotype, or observable characteristics. This interaction can lead to unexpected phenotypic ratios in offspring and can be categorized into various types based on the nature of gene interaction.

Several examples show the concept of epistasis.

1. Coat color in mice: The agouti (A) gene dictates coat color patterns in mice. The dominant allele (A) results in agouti fur, while the recessive allele (a) leads to solid-colored fur. However, another gene (C) is important for pigment production. Mice with the recessive allele (c) at this locus exhibit albinism irrespective of their genotype at the A locus. Hence, the C gene is epistatic to the A gene, as it can override the effect of the A gene.
2. Fruit color in summer squash: Fruit color in summer squash is controlled by two genes, W and Y. The dominant allele of W (W) produces white fruit, while the recessive allele (w) allows expression of the color determined by the Y gene. The Y gene has a dominant allele (Y) for yellow fruit and a recessive allele (y) for green fruit. The presence of a dominant copy of the W gene results in white fruit, regardless of the Y alleles. This illustrates dominant epistasis.
3. Flower color in peas: Flower color in peas is governed by two genes. One gene regulates the production of a pigment precursor (P), while the other controls the conversion of the precursor into a colored pigment (C). Dominant alleles (P and C) yield purple flowers, whereas recessive alleles (p and c) lead to white flowers. However, if both genes are recessive (ppcc), the plant cannot produce any pigment, resulting in white flowers. This is an example of duplicate recessive epistasis, in which both genes must dominate for the colored phenotype to occur.

Epistasis is a complex phenomenon with significant implications for trait development. Breeders can leverage this knowledge to cultivate new plant and animal varieties with desired traits. Additionally, understanding epistasis is increasingly essential in medicine, as researchers explore its role in complex diseases.

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